Electrical amplifiers are required in order to amplify signals with respect to voltage, current or power. For this, normally active elements, as transistors or electron tubes, are used. However, transformers can cause an amplification, too.
However, on account of the non-linearity of the characteristic transmission curve of these elements non-linear distortions are generated in the output signal, i.e. additional signal portions which are not present in the original signal. In order to reduce these non-linear distortions the so-called negative feedback is used according to which a portion of the output signal is fed back to the input with a phase shift of 180°. However, this method has substantial disadvantages with non-stationary signals since new signal falsifications are generated on account of different running times.